Slabs, Beams, Columns, Shear Walls & Transfer Structures Notes

Last reviewed 30 Sept 2026 · 6 min read

The load path in a station

The main design principle is a clear, continuous load path: every load — crowd, train, earth, wind, earthquake — must travel from where it is applied, through slabs to beams to columns and walls, and finally into the foundations. Stations have several features that break the regular grid: the track and platform layout, large open concourses, stair and escalator openings, and interchange levels. Structural design is largely the art of dealing with these interruptions.

Slabs

Types: RCC flat slab (with drops or capitals), beam-and-slab, waffle, prestressed (bonded or unbonded) for longer spans; ribbed / coffered slabs for heavy plant floors.
Design: span/depth limits for deflection; flexure, shear and — for flat slabs — punching shear at the columns; crack width limits (about 0.2–0.3 mm; smaller where water-tightness matters, as in the roof and base slab of an underground box).
Roof slab of an underground station: earth cover, road traffic, utilities and possible construction loads; waterproofing and drainage details; it acts as a prop to the walls (in a top-down construction it is built first).
Base (raft) slab: resists uplift and distributes the column and wall loads to the ground, designed as a slab on elastic foundation.
Openings for stairs, escalators and shafts require trimmer beams and additional reinforcement; the load path around openings must be checked.

Beams

RCC or PSC beams supporting slabs and forming frames with columns; shear is critical under heavy loads and near supports.
Deep beams when the span/depth ratio is small (below about 2 for simply supported spans): the strut-and-tie model is used rather than beam theory.
Platform edge beams carry the platform screen doors and tolerances at the edge next to the train; deflection and settlement limits are strict.
Torsion: in edge beams and in eccentrically loaded beams; closed stirrups and longitudinal steel.

Columns

Station columns are heavily loaded (many levels, plant loads) and slender in the long direction at concourse levels:

Design for axial load with uniaxial or biaxial bending, including the second-order effects of slenderness; use high-strength concrete (M50–M60) and steel to keep sizes small; spirals or closely spaced ties provide confinement in seismic zones.
Shape: circular columns are common in elevated stations because of their efficient use and appearance, and to reduce the obstruction for traffic below; rectangular columns in underground boxes.
Fire: cover and sizes meet the fire-rating (often 2–4 hours).
Construction: high-strength concrete with dense reinforcement requires good vibration; splices are staggered and located away from the high moment regions in seismic zones.

Shear walls and cores

Shear walls (in cores of stairs, lifts, and plant) resist lateral loads — wind and earthquake — and reduce the moments in the frame.

Design as cantilevers with flexure and axial load (interaction diagram), shear (horizontal and vertical steel), and boundary elements (confined ends) in seismic design.
Coupled walls with openings: the coupling beams need diagonal reinforcement for ductility.
Underground boxes: interior walls and diaphragm walls provide the lateral stiffness; the roof and floor slabs act as diaphragms that transfer horizontal force to the walls.
Openings and re-entrant corners cause stress concentrations; extra bars at corners.

Transfer structures

A transfer structure carries the loads of the members above to a different arrangement of supports below. Typical situations:

Viaduct pier over the station: the viaduct columns sit on transfer girders spanning the concourse below, where the column grid is different.
Column-free concourse: upper columns land on transfer beams or girders that span the open area.
Change of the column position between levels (for a plant room or a stair).
Podium or shopping mall above an underground station.

FormulaKey aspects of transfer girder design

Load path — very heavy point loads from columns above; shear and deep-beam action near the supports; the girder is often 2–4 m deep, so the strut-and-tie model and non-linear FEM verify it.
Prestressed (PSC) transfer girders reduce depth, control cracking and deflection, and carry huge loads; tendons are draped and the anchorage zones are detailed carefully.
Local bearing under the columns above, splitting and bursting reinforcement.
Deflection limits, because the deflection of the transfer girder affects the floors above: the stiffness of the supporting columns and the shear deformation of a deep member also matter.
Stage construction: the girder may be cast and stressed before the loads above are applied; the sequence of loading and prestressing is analysed.
Robustness: if one support is lost, the structure should not collapse progressively.

Deep-beam design. A simply supported beam with $L / D$ < 2 behaves as a deep beam: the strains are non-linear over the depth, the lever arm is about $0.6 L$ (for $L / D$ = 1) rather than the section depth, and the flexural steel should be distributed near the bottom; web reinforcement in both directions controls cracks.

Worked ExampleExample — a simple strut-and-tie check

A deep transfer beam of span $L$ = 6 m and effective depth 3 m carries a concentrated column load $P$ = 6000 kN at mid-span, supported on two columns.

Reaction at each support $R$ = 3000 kN. The lever arm $z \approx 0.7 \times$ depth ≈ 2.1 m.

Tie force (bottom bars) $T = \frac{P L}{4 z} = \frac{6000 \times 6}{4 \times 2.1} = 4286$ kN.

Required steel with $0.87 f_{y}$ ( $f_{y}$ = 500): $A_{s} = \frac{4286 \times 1 0 ^{3}}{0.87 \times 500} = 9853$ mm² — about 20 bars of 25 mm dia. in 2 layers.

The struts (inclined at about 55°) are then checked for compressive stress, and the nodes for bearing.

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